While there are some differences throughout the U.S., typically on the integrated grids of all resources (coal, nuclear, natural gas, oil, and renewables), biomass and geothermal units are often dispatched as base load and displace coal fired generation. Wind and solar units are generally dispatched as peaking units and displace natural gas.
The Electric Power Research Institute has an excellent technical paper explaining why CO2 emissions associated with coal-fired generation are significantly higher than the use of natural gas. EPRI's comparison basis is called the "carbon intensity" ratio and reflects:
The efficiency of power plant technology is measured by the unit's heat rate (i.e., the amount of Btu's required to produce 1 kWh of electricity). For example, the higher a unit's heat rate, the lower its efficiency will be. Conversely, the lower a unit's heat rate, the higher its efficiency (thus using less fossil fuel and producing less air emissions of CO2, NOx, and SO2 to generate 1 kWh of electricity).
As the above EPRI data reflects, when biomass energy displaces coal use (e.g., such as in biomass co-firing at an existing coal unit, or in a State like Kentucky where 87% of electricity generation is from coal) the CO2 reduction benefits can be almost twice as great than with a solar or wind unit that displaces a natural gas generating unit's dispatch on the grid.
For more information on this topic, you can go to our Quick Facts on Biomass Energy.